An important class of enzymes that has been the subject of extensive study is protein kinases which are involved in a majority of cellular signaling pathways affecting cell proliferation, migration, differentiation, and metabolism. Kinases function by removing a phosphate group from ATP and phosphorylating hydroxyl groups on serine, threonine and tyrosine amino acid residues of proteins in response to a stimulus such as environmental and chemical stress signals (e.g. osmotic shock, heat shock, ultraviolet radiation, bacterial endotoxin), cytokines (e.g., interleukin-1 and tumor necrosis factor alpha), and growth factors (e.g. granulocyte macrophage-colony-stimulating factor, transforming growth factor, fibroblast growth factor). Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease and hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find inhibitors of protein kinase that are effective as therapeutic agents.
Aurora kinase is a family serine/threonine kinases that are essential for cell proliferation. The three known mammalian family members, Aurora-A (also referred to as Aurora-2, Aur-2, STK-15), Aurora-B (also referred to as Aurora-1, Aur-1 and STK-12) and Aurora-C (also referred to as STK-13), are highly homologous proteins responsible for chromosome segregation, mitotic spindle function and cytokinesis. (Bischoff, J. R. & Plowman, G. D., Trends in Cell Biology 9:454, 1999; Giet R. and Prigent, C. Journal of Cell Science 112:3591, 1999; Nigg, E. A., Nat. Rev. Mol. Cell Biol. 2:21, 2001; Adams, R. R. Carmena, M. and Earnshaw, W. C., Trends in Cell Biology 11:49, 2001). Aurora kinase expression is low or undetectable in resting cells, with expression and activity peaking during the G2 and mitotic phases in cycling cells. In mammalian cells, proposed substrates for Aurora kinase include histone H3, a protein involved in chromosome condensation, and CENP-A, myosin II regulatory light chain, I protein phosphatase 1, TPX2, all of which are required for cell division. Aurora-A plays a role in the cell cycle by controlling the accurate segregation of chromosomes during mitosis and misregulation thereof can lead to cellular proliferation and other abnormalities.
Since its discovery in 1997 the mammalian Aurora kinase family has been closely linked to tumorigenesis due to its effect on genetic stability. Cells with elevated levels of this kinase contain multiple centrosomes and multipolar spindles, and rapidly become aneuploid. Indeed, a correlation between amplification of the Aurora-A locus and chromosomal instability in mammary and gastric tumours has been observed. (Miyoshi, Y., Iwao, K., Egawa, C., and Noguchi, S. Int. J. Cancer 92:370, 2001; Sakakura, C. et al. British Journal of Cancer 84:824, 2001). Moreover, Aurora-A overexpression has been shown to transforms rodent fibroblasts (Bischoff, J. R., et al. EMBO J. 17:3052, 1998).
The Aurora kinases have been reported to be overexpressed in a wide range of human tumours. Elevated expression of Aurora-A has been detected in over 50% of colorectal, ovarian and gastric cancers, and in 94% of invasive duct adenocarcinomas of the breast. Amplification and/or overexpression of Aurora-A have also been reported in renal, cervical, neuroblastoma, melanoma, lymphoma, bladder, pancreatic and prostate tumours and is associated with aggressive clinical behaviour. For example, amplification of the aurora-A locus (20ql 3) correlates with poor prognosis for patients with node-negative breast cancer (Isola, J. J., et al. American Journal of Pathology 147:905, 1995). Aurora-B is highly expressed in multiple human tumour cell lines, including colon, breast, lung, melanoma, kidney, ovary, pancreas, CNS, gastric tract and leukemias (Tatsuka et al 1998 58, 4811-4816; Katayama et al., Gene 244:1). Also, levels of Aurora-B enzyme have been shown to increase as a function of Duke's stage in primary colorectal cancers (Katayama, H. et al. Journal of the National Cancer Institute 91:1160, 1999). Aurora-C, which is normally only found in testis, is also overexpressed in a high percentage of primary colorectal cancers and in a variety of tumour cell lines including cervical adenocarinoma and breast carcinoma cells (Kimura, M., et al., Journal of Biological Chemistry 274:7334, 1999; Takahashi, T., et al., Jpn. J. Cancer Res. 91:1007-1014, 2000).
Based on the known function of the Aurora kinases, inhibition of their activity will disrupt mitosis leading to cell cycle arrest halting cellular proliferation and therefore will slow tumour growth in a wide range of cancers.